Efforts to understand the genetic mechanisms that underlie congenital inner ear defects are now focused on both the normal processes of development and on the cascade of events that can arise in response to a single gene mutation. Because the generation of animals with a targeted gene defect has become commonplace in mice, this species has emerged as a powerful model system in which to pursue the relationship between gene expression and morphogenesis. Counteracting the power of mouse genetics is the inaccessibility of the mouse embryo throughout the critical stages of organogenesis, which begins in utero during the second week after fertilization. Because of its inaccessibility, one key experimental approach, fate mapping of an organ primordium, is rarely performed in the mouse embryo. At the present time, it is not known in any species which parts of the otic placode and early otic vesicle give rise to the different parts of the ear. Such information can be obtained by fate mapping, and is critical for interpreting how gene expression domains get converted into patterning information. The ability to superimpose the expression domains of the greater than 40 genes expressed in the ear with a high-resolution fate map of the otocyst could have a major impact on the field of ear development. Furthermore, fate mapping a mouse inner ear that is abnormal due to a known genetic mutation promises to provide insights that are simply not possible by descriptive analysis alone. For example, it may provide information about whether a specific genetic mutation is causing a change in cell fate that can explain the mutant phenotype. The first specific aim is to fate map the mouse otic cup in both the wild-type mouse and in a mouse mutant, kreisler, that develops with gross abnormalities in the inner ear. This will be accomplished by small focal injections of lipophilic carbocyanine dyes directed into the developing ear epithelium of mouse embryos grown in culture. The labelled cells will be mapped to see where they reside after otic vesicle closure (after 24 hours) or otic vesicle morphogenesis (after 48 hours). The second specific aim is to pilot methods to facilitate focal gene transfer into the otocyst of the cultured mouse embryo. This will be accomplished by injection of retrovirus stocks or small numbers of retrovirus-producing cells. The study will be performed with green fluorescent protein as a marker for the purpose of piloting the methods. The long-term goal driving the development of an in vitro paradigm for mouse ear development is that it may lead to intervention strategies (such as virus-mediated gene transfer) designed to rescue the inner ear defects arising from known genetic mutations. If the mouse whole embryo culture paradigm proves successful, its impact is likely to extend far beyond the proposed studies, given that the number of mutant and knockout mice generated as potential models of human deafness genes will continue to rise.